For industrial and commercial applications the preferred roof and floor joist is a top chord bearing joist. In North America the top chord bearing joist market is predominantly serviced by the open web steel joist (OWSJ) market that is regulated by the Steel Joist Institute (SJI). The top chord joist provides an excellent method for erection when a crane is used, as top chord bearing joists passively stay in place by gravity. A problem with the present art is that the designs require an abundance of parts and considerable man hours to produce. The OWSJ is difficult to customize for the many alternative conditions that arise on construction projects today. Present top chord bearing joists as described in the SJI specifications are typically built using hot rolled steel shapes; however, some OWSJ designs have elements that are cold rolled shapes. There are top chord bearing joists using special cold formed shapes that are arranged in a manner similar to the OWSJ, so these proprietary OWSJ projects also have abundant parts and require abundant man hours to produce. The top chord bearing joists specified in SJI and the special shaped joists both require approximately 6 to 10 man hours per ton to manufacture and require additional man hours to customize.
Typical standard joists as identified in the Steel Joist Institute (SJI) specifications have top and bottom chords that are angle sections and the webs are round bars or cold formed U shapes or crimped angles; shoes for end bearing are fixed to the top chords at the ends, typically by welding. These joists are customized to suit the conditions of each project. When OWSJ's are used for sloping conditions, the shoes are typically drawn by a draftsman, arranged to fit the desired angle by a fitter in the shop and then welded. Installing sloped shoes can be very expensive. Concentrated loads often require the need of engineering to satisfy special loading conditions. On a 40 ft long joist there will be approximately 44 pieces. Many pieces are different sizes and weights; the pieces are custom cut on a saw line for each project. In today's industry joists are produced with a high factor of labour cost, from 6 to 10 man hours a ton depending on location, support infrastructure, plant capabilities and product mix.
Some cold formed joist systems are available on the market that have very similar assembly methods to that of the OWSJ. The available systems often have special shaped chords and web members. A cold formed top chord bearing joist system maintains a high quantity of parts utilization and associated man hours to assemble. Most of the available cold formed joist systems are difficult to customize similar to the OWSJ products described in SJI.
FIG. 1 and FIG. 2 shows prior art open web steel joists. FIG. 2 is a sketch of an alternate cold formed top chord bearing joist as described in U.S. Pat. No. 6,519,908 filed 27 Jun. 2000 and titled “Structural member for use in the construction of buildings”.
In the past many innovative steel joist designs have been developed and introduced to the market. The market is demanding in terms of performance requirements to suit alternative building design types, therefore these products require significant customization for each project. The present art of OWSJ designs, such as those shown in FIG. 1 and FIG. 2, are difficult to adapt to the alternative project conditions and design protocols. Customizing alternative joist designs to suit different conditions can be expensive. A top chord bearing joist that can be manufactured with the fewest number of pieces and require the smallest amount of physical modification, yet suit all of the alternative conditions would be highly desirable.
Therefore, a new and improved way to provide top chord bearing joists would be to provide a joist that reduces labour hours, reduces material use and is easy to customize for the many alternative project conditions.